Project Summary: The active form of vitamin D (1,25(OH)2D) regulates events in bone, kidney and intestine to control whole body Ca metabolism and influence the development of osteroporosis. Several lines of evidence suggest that the role of Ca absorption in osteoporosis development requires additional attention: Ca absorption efficiency is reduced with aging and in post-menopausal women with fractures; low fractional Ca absorption has been associated with increased hip fracture risk in post-menopausal women, and intestinal resistance to the action of 1,25(OH)2 D develops in the elderly and in post-menopausal women. The long-term goal of my research is to understand the cellular mechanisms causing low fractional Ca absorption and reduced intestinal vitamin D action that contribute to osteoporosis. Many aspects of the models proposed to explain vitamin D-regulated Ca absorption have not been tested. This proposal reflects the evolution of our mechanistic understanding of vitamin D action in the enterocyte and our interest in translating fundamental research findings into the complex physiology of whole body Ca metabolism. Our specific aims are: (1) To assess the importance of VDR location and level in the control of intestinal Ca absorption. We will use genetically modified mice to test if high intestinal VDR can prevent age-associated calcium malabsorption and intestinal vitamin D resistance (subaim1a) and we will determine whether the deletion of VDR specifically from the ileum, cecum, and colon can alter whole body calcium metabolism (subaim1b), (2) To determine whether the apical membrane Ca channel TRPV6 is essential for intestinal Ca uptake and absorption. We will use genetically modified mice to determine whether intestinal TRPV6 can recover the VDR null phenotype and prevent loss of Ca absorption with aging, (3) To determine the factors controlling vitamin D-mediated gene activation in enterocytes. We will conduct cell and animal studies to determine the active role RXR has in VDR mediated gene transcription (subaim 3a) and cell studies to assess the role that intranuclear VDR movement and promoter on-off kinetics plays in vitamin D-mediated gene transcription (subaim 3b). We are uniquely positioned to test these hypotheses and to expand our understanding of how vitamin D regulated Ca absorption helps protect bone health. Our work will provide preclinical evidence for strategies to optimize Ca absorption and prevent osteoporosis.